Microbes and food 1. Menu - you are what you eat
1.16 Bread
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What is it?
It is a staple part of the diet in many countries. It has been made for thousands of years. Originally bread was unleavened, because it was made without a raising agent. It was flat and very hard. You can see 4,000 year-old samples of unleavened bread in the British Museum.
Yeast photomicrograph
Picture 1.16a Yeast grows by a process of cell division, which we call budding.
How is it produced?
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Bread is made from cereal grains ground into flour. Leavened bread is softer due to the action of yeast, which is a fungus. Yeast (Saccharomyces cerevisiae) contributes both to the texture and flavour of bread. A dough is made by mixing yeast with flour, salt and water. The yeast ferments sugars in the mixture to make alcohol and bubbles of carbon dioxide. The gas gets trapped in the sticky proteins of the dough and causes it to rise, whilst the alcohol is converted to compounds which impart taste to the bread as it is baked in the oven. Bread-making is an aerobic process and the main products of the fermentation are carbon dioxide and water, rather than alcohol, as produced in beer and wine manufacture.

Roll over the headings (starting with Mixing) to find out more.

Animation of rising bread
Picture 1.16b The bread takes about 30 minutes to rise.

Baking bread photo
Picture 1.16c Baking bread.
The flour, water, salt and yeast are mixed to make the dough.
The dough is kneaded to mix the ingredients thoroughly.
The dough is shaped and left in a warm place. Enzymes from the flour convert some of the starch into sugars. The yeast ferments the sugar, producing carbon dioxide gas and alcohol. The gas forms bubbles that make the dough rise. Often, the dough is then flattened and allowed to rise a second time.
Baking at 180 ºC kills the yeast, stops fermentation, makes the alcohol evaporate and escape and it browns the outside to form a crust.
How does it spoil?
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When bread comes out of the oven it is usually sterile. Also its dry surface prevents the rapid growth of any microbes or their spores in the environment that might contaminate it. However if bread is kept in a closed container such as a plastic bag it will tend to go mouldy in time. This happens because water that evaporates from inside the bread is trapped and makes the surface moist, providing good conditions for microbial growth. Bread is more likely to go mouldy if it is sliced and packaged as it can become contaminated by the slicing and wrapping equipment as well as from the air; microbes will also be distributed throughout the product. Bread occasionally spoils due to bacterial growth, either by the growth of bacilli from spores that survive baking or by contamination with Serratia marcescens which turns the bread red.
Mouldy bread
Picture 1.16d Mouldy bread and red bread .

Preservatives such as sorbic or proprionic acids are sometimes added to bread. They slow down the growth of microbes such as yeast and moulds.

Bread should be stored in a clean, dry container that allows the free flow of air. The air will stop evaporating moisture from inside the bread making the surface damp and encouraging microbial growth. Bread should be eaten within a few days.